The LTE standard is capable of providing services for device positioning wherein the position of User Equipment (UE) in the network, such as a mobile device, can be determined. The network comprises a location server for the provision of information to assist in position determination and/or to compute the device position.
Calculation of the device position can be divided into two classes:                In the first class, known as UE-based, the UE computes its own position and reports the position to the location server. The location server may provide the UE with information (known as assistance data) to improve the positioning accuracy or reduce the time taken to determine position by the UE.        In the second class, known as UE-assisted, the UE does not compute its own position but performs measurements that are used by the location server to compute the UE position. The network may provide assistance data to ease the measurement undertaken by the UE.        
For both classes, communication between the location server and the UE takes place using well defined exchanges of message(s).
Initiation of positioning may be:                Network initiated: the network requests the UE position. This occurs, for instance, during an emergency session after a user has dialed 911 in the US.        UE initiated: the UE wants to know its position: this occurs for instance when a user is using its device to know its location.        
In 3GPP systems, the positioning system relies on:                3 positioning technologiesGlobal navigation satellite system (GNSS), observed time difference of arrival (OTDOA) and enhanced cell ID (ECID). In known systems, GNSS may operate in both UE-based and UE-assisted classes, while OTDOA and ECID are UE-assisted only.        The LTE positioning protocol (LPP) and/or secure user plane location (SUPL) standards for the communications protocol.        
These protocols define how the location server and the UE communicate to exchange information related to positioning.
GNSS
GNSS is a technology that is well known to work in a UE based way (for instance in a car) but it can also work in a UE assisted way.                When UE-based, the UE computes the propagation delay in relation to the GNSS satellites in view and also decodes the signals transmitted by the satellites to retrieve satellite position (the ephemeris). The acquisition time for the ephemeris is typically at least 18 s and can be much longer (several minutes). Therefore, schemes have been defined to assist computation by the UE. For example, the network can provide ephemeris information to the UE to reduce the acquisition time significantly (to a few seconds). Further, almanacs may be provided to avoid the GNSS receiver of the UE having to search for satellites in vain (for example if a satellite is broken and therefore not providing GNSS information).        When UE-assisted, the UE simply measures the propagation delay of a list of satellites provided by the network. The UE knows the GNSS satellites propagation delay but not the ephemeris and therefore only the network can compute the position. In addition, the network may provide to the UE the expected phase and Doppler range of each satellite. This aids the UE to reduce the satellite search space and hence reduce the time taken to make the measurements of the satellites propagation delays.Drawbacks of GNSS Positioning        
FIG. 1 summarizes a typical GNSS positioning session in 3GPP for UE-based calculation. FIG. 2 summarizes a typical GNSS positioning session in 3GPP for UE-assisted calculation. In all figures, optional message exchanges are depicted using dashed arrows.
UE-based GNSS Positioning:
In FIG. 1, when the UE (12) computes its position, the UE may decide to carry out the calculation on its own without a position request (11) from the location server (10). The UE may also omit to report its position to the server (13).
Drawbacks in the case of UE-based GNSS positioning:
The UE must transmit an assistance request message (14) before receiving assistance data (15). This will cause energy consumption by the UE. An example where this assistance request is mandatory is when the GNSS device on the UE wants to compute its position on its own without a location server request (11).
If the UE is not connected, it will first need to establish LTE connectivity before being able to transmit the assistance request. This will cause further energy consumption as several messages have to be exchanged between the local eNB and the UE to establish an LTE connection as would be understood.
UE-assisted GNSS Positioning
In FIG. 2, in UE-assisted positioning, subsequent to a measurement request (15), the location server (10) may send assistance data (16) before an assistance request (14) from the UE (12) in which case an assistance request (14) will not be requested by the UE (12). When a UE-assisted session finishes, the location server may or may not transmit the computed position to the UE (18).
Drawbacks in the case of UE-assisted GNSS positioning:
The assistance information (16) must be provided to the UE and hence may cause energy loss at the UE if assistance information is not transmitted spontaneously by the network. This is generally the case if the location session is network initiated but in this case the device may not know its own position at the end of the session. If the session is initiated by the UE, then it will necessary request assistance (14) which will consume energy to transmit the corresponding message. This use case can be used to offload the complex position computation from the UE to the server or to perform fewer measurements at the UE and speed up the measurement duration owing to use of the assistance information.
For both UE-based and UE-assisted GNSS schemes, the following drawbacks are evident:                Assistance information is always unicast (sent to a unique UE) while it could be broadcast to all the UEs within a cell. For instance the ephemeris could be broadcast to all the UEs within a cell (as the satellite positions are absolute and independent of the cellular network).        Assistance information is sent for each positioning session even if the assistance information has not changed (for example the ephemeris can last typically around a couple of hours while almanac can be valid for several days).        Assistance information wastes energy if the UE has to request it.OTDOA        
OTDOA is a technology that has been introduced with LTE. It can provide more accurate positioning compared to GNSS, and it has the advantage of being able to work seamlessly indoors and outdoors (while GNSS is known to face issues with indoor or urban environments). OTDOA is a technology that works only in a UE-assisted way in 3GPP.
OTDOA Operates as Follows:
The eNB sends positioning pilot signals, known as PRS (positioning reference signal) inside the LTE radio signal according to a defined pattern (period and position of the PRS).                The PRS can be used by the UE (12) to measure the relative propagation delay between the eNB and the UE with good accuracy, known as RSTD (reference signal time difference).        When a positioning session occurs, the location server (10) provides the list of eNB sending PRS in the vicinity of the UE (12) as well as the PRS patterns that are used by each eNB. The location server also provides to the UE, an estimate and expected window for the propagation delay for each eNB. That way, the UE can measure the RSTD around this estimate using a reduced search window and hence save processing resources. All this information is known as OTDOA assistance and it is mandatory for the UE to perform the RSTD estimation.        The RSTD information is reported to the location server.        The location server knows the eNB position and can then estimate the UE position (12) through hyperbolic equations using the relative time difference τ3-τ1 and τ2-τ1 as illustrated in FIG. 3 (picture from document “Observed Time Difference Of Arrival (OTDOA) Positioning in 3GPP LTE” by Sven Fisher—Jun. 6, 2014—Qualcomm.Drawbacks of OTDOA        
Typical OTDOA positioning session in 3GPP is summarized in FIG. 4.
In known systems, OTDOA works only in a UE-assisted way. The UE only reports RSTD measurements (19) and cannot compute its own position as this would require knowledge of the position of the eNodeBs.
The drawbacks of such a scheme:                Assistance information (20) is mandatory for OTDOA positioning resulting in the drawbacks listed in the next paragraph.        The UE is unable to compute its own position which prevents some use cases. For instance, OTDOA positioning cannot be used to replace GNSS on a watch for runners.        The device must use energy to transmit the RSTD report (19).        
The assistance scheme has 2 additional drawbacks:                Assistance information is always unicast while the PRS and expected RSTD with the maximum RSTD uncertainty could be broadcast to all UEs in a cell (the uncertainty modeling the variation of the RSTD per UE)        Assistance information is sent for each positioning session even if the assistance information has not changed. The sending is, therefore, frequently redundant information as the assistance information is not expected to change unless the UE has moved significantly since the last positioning session occurred or the last positioning that occurred is so old that it is safer to refresh it. For instance, the resending of assistance data could be limited to:                    Each time the UE has carried out a handover to a new cell            Or less frequently, each time the UE experiences a change of tracking area                        
As would be understood, a tracking area is a group of cells, each cell corresponding to an eNodeB. All eNodeB within a tracking area are connected to a single MME (Mobility Management Entity) which is a server connected to all eNB of the tracking area. MMES are interconnected together to ensure calls continuity and tracking of the UE.
Communication Protocol Used for Positioning
The protocols used for positioning in 3GPP are:                The LTE positioning protocol (LPP).        Secure user plane location (SUPL) and non-access stratum (NAS) protocol for the transport layer.        
The LPP protocol is a protocol dedicated to positioning that is defined in LTE Positioning Protocol (LPP) (3GPP TS 36.355)—ETSI TS 136 355 V11.2.0 (2013 April). Relevant parts of LPP are described herein as appropriate.
SUPL and NAS are considered, without restricting the scope of this disclosure. The disclosure applies equally to both SUPL and NAS or any other transport protocol. SUPL and NAS are considered as transport layer, without restricting the scope of this disclosure. NAS is one layer of the control plane in LTE. It is mostly used as any other control plane layer to define how transmission occurs and to establish the user plane flow. A NAS message typically contains a description of the transmission flow. However, NAS also has the capability to convey information messages which can be for instance SMS or LPP messages. NAS acts in this case as a transport protocol and if the message is an LPP message, this is what is usually called LPP over control plane (a.k.a. LPP over NAS). On the other hand, information messages (including Internet messages) are generally transferred on the user plane (i.e. at the Internet protocol (IP) level). This is specifically the case when LPP is transmitted in a SUPL session. This is usually called LPP over the user plane or LPP over SUPL.
LPP is a positioning protocol that operates on a per positioning session basis as shown in FIG. 5. Each time a position/positioning measurement is required, a new LPP session is started. This session will happen either:                At the user plane level (above IP) using the SUPL protocol as a transport layer.        At the control plane level using the NAS as a transport layer.        
In both cases, the UE has to be in connected mode so that the LPP exchanges can take place.
With reference to FIG. 5, and without any loss of generality, a positioning session typically operates as follows when the session is initiated by the location server:                The location server (10) sends a request to the UE (12) to provide its measurement capabilities, i.e., the positioning technologies it supports (51).        The UE answers the server request by providing its capabilities (52).        Then, the actual positioning phase starts.        Location server provides assistance information (53) to the UE which can be:                    OTDOA assistance information            GNSS assistance information for UE-based session (for example ephemeris, almanacs, etc)            GNSS assistance information for UE-assisted session (satellites to measure with ranges for satellite phase and Doppler search)                        Location server then requests the location information (54).        UE answers the server requests and provides either the measurements and/or the position (55).        The session is ended.        
When the positioning session is initiated by the UE, the session typically occurs as follows:                UE may provide its capabilities spontaneously.        UE requests assistance information.        UE may provide location information to the server or keep it for its own use.        The session is ended        
In addition to the messages of FIG. 5, the LPP protocol also supports duplicate detection, message acknowledgements, retransmissions, error handling, and abort procedures. This additional functionality can increase the number of exchanged messages between the location server and the UE (downlink and uplink).
Problems to be Solved
Battery Life
Power consumption is important in most embedded systems as it will directly determine the battery life of, for example, a UE. For UEs performing positioning, the power consumption will depend mostly on:                The intrinsic power consumption of the positioning technique (OTDOA, GNSS, etc. . . . ) which mostly depends on the duration to acquire the positioning signals and compute the measurements or position.        The number of downlink and uplink messages received and sent during the positioning session. It is noted that uplink transmission from a UE requires much more energy than receiving a downlink message.        
Therefore, it is desirable to reduce the number of messages dealt with by the UE when performing positioning calculations.